1. Field of the Invention
The present invention relates to a novel radiographic intensifying screen.
2. Description of Prior Art
In medical radiography, a radiographic image of the organ of patient is obtained by recording a pattern of X-rays having passed through the patient on a photographic material (silver halide photographic material) in which at least one photosensitive silver halide emulsion layer is provided on a transparent support by coating. Although the pattern of X-rays having passed through the patient can be obtained using a silver halide photographic material alone, a radiographic intensifying screen is usually employed in combination with the photographic material so as to avoid exposure of the objective human body to a large dose of X-rays. In a radiographic intensifying screen, provided on a support is a phosphor layer which absorbs X-rays and then converts them into the visible light to which the photographic material is sensitive. Therefore, the sensitivity of the system for X-ray photography can be remarkably improved by the use of a radiographic intensifying screen.
As a method for further enhancing the sensitivity of the system for X-ray photography, proposed was the method in which a photographic material having photographic emulsion layers provided on both sides, namely, a silver halide photographic material having silver halide photosensitive layers provided on both of the front and the back sides of the support, is used, and radiographic intensifying screens (which are sometimes referred to as "intensifying screens") are placed on both sides of said material to obtain an X-ray photography in this arrangement. At present, this method is generally used in most cases. This method has been proposed in view of the fact that sufficient amount of X-rays is not absorbed by the use of only one radiographic intensifying screen. In fact, even if content of the phosphor in one intensifying screen is increased in order to increase absorption of X-rays, the photographic material placed in contact with said intensifying screen receives very diffuse visible light emitted by said screen because the visible light converted within the phosphor layer is scattered and reflected in the phosphor layer having an increased thickness which is caused by the increase of the phosphor. In addition to that, the visible light converted within the deep area of the thick phosphor layer hardly comes out of the phosphor layer. Therefore immoderate increase of the phosphor content does not further increase the effective visible light emitted by the intensifying screen.
By the X-ray photographic system (i.e., radiography) using two intensifying screens each of which has a phosphor layer of adequate thickness, the total absorption of X-rays can be increased and the visible light converted in the intensifying screen can be efficiently used.
Commercially available are a variety of radiographic intensifying screens having different sensitivities in a wide range; for example, from the type of low sensitivity such as a screen that emits weak luminescence and gives an image of high sharpness (such screen has a relatively thin phosphor layer) to the type of high sensitivity such as a screen that emits high luminescence and gives an image of low sharpness (such screen has a relatively thick phosphor layer).
However, although various types of silver halide photographic materials having photographic emulsion layers provided on both sides are commercially available, their sensitivities vary within a narrow range. In fact, the sensitivities of the most photographic materials are only twice or three times as much as that of the least one (sensitivity of the lowest sensitive material is used as standard).
Employable combinations of silver halide photographic materials and intensifying screens used for radiography are not particularly restricted. However, when highly sensitive radiography is required (for example, in the case of radiography for lumbar vertebra, angiography for head or enlargement radiography), a combination of an intensifying screens of high emission and a silver halide photographic material of standard type or of high sensitivity is generally employed. In contrast, when high image quality is needed (for example, in the case of simple radiography of chest, gastric radiography using contrast medium, or radiography of bones), a combination of intensifying screens providing high sharpness and a silver halide photographic material of standard type is generally employed. While a combination of intensifying screens of high sensitivity and a silver halide photographic material gives an image of low sharpness (namely, low image quality), a combination of intensifying screens providing high image quality and a photographic material shows low sensitivity.
Previously, in order to establish an X-ray photographic system having excellent characteristics in balance between sensitivity and image quality, various studies have been continuously made. For instance, although a combination of intensifying screens having phosphor layers of calcium tungstate (which emits blue luminescence) and a silver halide photographic material which is not spectral sensitized (e.g., a combination of HIGH SCREEN STANDARD and RX [each of which is a tradename of Fuji Photo Film Co., Ltd.]) has been formerly used, a combination of intensifying screens having phosphor layers of terbium activated rare earth oxysulfide phosphor (which emits green luminescence) and a silver halide photographic material which is orthochromatically spectral sensitized (e.g., a combination of GREENEX 4 and RXO [each of which is a tradename of Fuji Photo Film Co., Ltd.]) has been recently used. The latter combination gives an image improved in both sensitivity and image quality.
A silver halide photographic material having photographic emulsion layers provided on both sides has a defect that the image quality is likely deteriorated by "crossover light". The word "cross-over light" means the visible light which is emitted by each intensifying screen placed on each side of the photographic material and then passes through the support of the photographic material (thickness of the generally used support is 170-180 .mu.m) and finally reaches the photosensitive layer provided on the opposite side to deteriorate the image quality (particularly, in sharpness).
In order to reduce the cross-over light, various methods have been proposed. For example, U.S. Pat. Nos. 4,425,425 and 4,425,426 disclose inventions in which a spectral sensitized emulsion including tabular silver halide grains having a high aspect ratio is used as a silver halide photographic emulsion. According to these patented inventions, the cross-over light can be reduced to 15-22% level. Besides that, U.S. Pat. No. 4,803,150 discloses an invention in which a dye layer comprising fine crystalline dye particles and being decolorizable by the developing process is provided between the support and the photosensitive layers in the silver halide photographic material. It is described that cross-over light can be reduced to 10% level or less by the invention.
On the other hand, various attempts to provide an X-ray photographic system showing excellent characteristics in balance between the sensitivity and the image quality have been made by setting a combination of intensifying screens and a silver halide photographic material (which has photographic emulsion layers on both sides) under particular conditions. For example, Japanese Patent Provisional Publication Nos. 2(1990)-266344 and 2(1990)-297544 and U.S. Pat. No. 4,803,150 disclose an X-ray photographic system: in which the light property (sensitivity) of the front side combination (consisting of an intensifying screen placed on the side to be exposed [i.e., front screen] and a photosensitive layer [i.e., front photosensitive layer]) is made to differ from that of the back combination (consisting of an intensifying screen placed on the opposite side [i.e., back screen] and a photosensitive layer [i.e., back photosensitive layer]), and the contrast given by the former combination is made to differ from that given by the latter. In addition to that, Photographic Science and Engineering, vol. 26 (1982), No. 1, pp. 40, describes experiments about the combinations of intensifying screens and a silver halide photographic material each of which is available from 3M Co. The experiments show the result that a combination of Trimax 12 (tradename of intensifying screen commercially available from 3M Co.) and XUD (tradename of silver halide photographic material commercially available from 3M Co.) gives better NEQ (signal/noise ratio of output) than the combination of Trimax 4 (tradename of intensifying screen commercially available from 3M Co.) and XD (tradename of silver halide photographic material commercially available from 3M Co.), while the former is at the same level of the latter with respect to sensitivity and sharpness (MTF). It further describes that the above result is caused by the facts that XUD exhibits higher sharpness than XD and that Trimax 12 absorbs more amount of X-rays than Trimax 4.
As is described above, various methods have been proposed to establish X-ray photographic system showing excellent characteristics in balance between the sensitivity and the image quality. However, previously proposed radiographic image forming methods still do not satisfy the sensitivity and the image quality required, for instance, for X-ray photographic system to be used for diagnosis of stomach or chest. Actually, while it is very important to observe veins in lung to the minutest details in diagnosis for chest, satisfactory observation can not be made by the known X-ray photographic systems. Since the known systems often give radiographic images having poor contrast of the vein image, image qualities are deteriorated in respect of grains, or blur. Further, in diagnosis for stomach, diagnostically satisfactory radiographic images in well picturing the gastric wall cannot obtained.
Needless to say, if other conditions are neglected, a radiographic image of high quality can be obtained by a combination of a silver halide photographic material of low sensitivity and radiographic intensifying screens of low sensitivity. However, if such low-sensitive combination is used, the exposure (dose) of X-rays applied to human body should be naturally increased. Therefore, such combination is not practically applicable, and particularly, in the case of group examination in which it is required to reduce the dose of X-rays to be applied as much as possible.